1. Field of the Invention
This invention relates to a dust collection apparatus for collecting dust, such as paper dust or powder, which occurs during an operation of a printing press or the like.
2. Description of the Related Art
This type of dust collection apparatus is typified most by a dust collection apparatus in a sheet-fed rotary press. That is, a sheet-fed rotary press applies printing to paper cut to a predetermined size. FIG. 8 shows a sheet-fed rotary press, in which sheets fed from a feeder (not shown) and printed by a printing section of a printing unit are transported to a delivery 100, and stacked in layers on a pile board 101. The sheets 102 stacked on the pile board 101, which have just undergone printing, have ink on their sheet surface insufficiently dried. Since the sheets 102 are stacked, one after another, in such an insufficiently dry state, so-called blocking, in which an image printed on the sheet 102 is transferred to the back of the sheet 102 placed directly above the lower sheet 102, occurs under the own weight of the sheet 102, or because of a slight impact caused by a falling sheet 102. This blocking phenomenon contaminates the back of the sheet 102, and peels off the ink applied onto the printed surface. Hence, the commercial value of printing products is markedly diminished.
To prevent this event, the printed surface of the sheet 102 is sprayed with a powder by a spray nozzle 103, as shown in FIG. 9, immediately before the sheet 102 is placed on the pile on the pile board 101, whereby a clearance corresponding to the particle diameter of a powder 104 is formed between the sheet 102 and the sheet 102, as shown in FIG. 10. The powder 104 is mainly composed of corn starch, and its particles have diameters of about 10 to 30 μm. On the other hand, the film thickness of the ink 105 printed on the sheet surface is about 1 to 2 μm. Thus, the particles of the powder ensure sufficient clearance. The powder spraying device disclosed there is an indispensable device for printing by a sheet-fed rotary press, unless a special device, such as an ultraviolet (UV) dryer, is provided.
Powder spraying by the spray nozzle 103 is performed for the sheet 102 being transported by a gripper device 107 (composed of a gripper 107a and a gripper pad 107b) of a delivery chain 106 in the delivery 100 (see FIGS. 8 and 9). In this case, the transport speed of the sheet 102 is about 3 m/s, and the distance between the spray nozzle 103 and the sheet 102 during powder spraying is about 0.1 m. In powder spraying work carried out at such a transport speed of the sheet 102 and over such a spraying distance, it is impossible for 100% of the sprayed powder 104 to adhere to the sheet surface. Normally, the amount of the powder 104 adhering to the sheet 102 is in the order of 10 to 30%, at most, based on the total amount of the sprayed powder 104. The remaining 70 to 90% of the sprayed powder scatters over the surroundings of the powder spraying device.
The scattered powder not only harms the work environment, but also deposits on stays, etc. within the printing press over time. The deposited powder cannot support its own weight, and collapses, falling as a lump over the printing product, thereby causing a printing trouble called “lumpy deposits.” The lumpy deposits occur abruptly, and are thus difficult to find, for example, by product inspection. It is extremely difficult to eliminate this trouble completely. For the purpose of preventing the lumpy deposits, a sheet-fed rotary press is furnished with a dust collection apparatus for sucking and removing the scattered powder.
The dust collection apparatus is of a common type called a bag filter. This collection apparatus has a dust collector body housing a blower, a filter, etc., and a suction duct leading from the dust collector body. The suction duct is disposed in place within the printing press.
A conventional technique for enhancing the dust collecting effect of the dust collection apparatus is disclosed in Japanese Utility Model Registration No. 2578195 (hereinafter referred to as Patent Document 1) According to a dust collection apparatus disclosed there, the neighborhood of a spray nozzle is surrounded by a shielding plate and a brush, and a powder is collected from within a range surrounded by the shielding plate and the brush so that air with a high dust concentration can be sucked. By so doing, the efficiency of the dust collection apparatus is increased.
With the above-described conventional apparatus, a clearance as large as 100 mm, at the smallest, must be formed between the shielding plate and a sheet guide, in order to avoid interference between the delivery gripper device and the shielding plate. The powder leaks through this clearance. To overcome this problem, the brush, which can be instantaneously rendered upright or lying, is used to close the clearance.
The brush, however, has been ineffective in that it cannot shut off the aforementioned range completely. Along the path of the delivery chain, in particular, a strong airflow (see Karman vortices α, β in FIG. 7) is created by the continuous running of a gripper bar 108 (composed of a gripper shaft 108a and a gripper pad shaft 108b) on which the gripper device 107 is mounted, as shown in FIG. 7. Borne by this airflow, a large amount of surplus powder is carried beyond the range surrounded by the shielding plate and the brush. To enhance the shielding effect, bristles of the brush may be elongated to decrease the clearance between the brush and the sheet guide. Too small a clearance, however, may result in the contact of the brush with the sheet surface, damaging the printed surface.
As shown in FIG. 8, the surplus powder is borne by the airflow created by the continuous travel of the gripper device 107 and the gripper bar 108, and is transported to an upper portion of the delivery 100. At the upper portion of the delivery 100, the surplus powder is blown downward by an air blower 110 comprising fans or the like for dropping the sheet 102 onto the pile board 101. Then, the surplus powder is flown outside through a lower opening portion of the delivery 100 (see downward arrows in FIG. 8). This flow of the surplus powder cannot be eradicated by the apparatus disclosed in the aforementioned Patent Document 1.